First Principles Study Of Several Catalysts Towards Nitrogen Cycle

Nitrogen is of great importance to the ecosystem as the main component of proteins and cells.The inert nitrogen(N₂)and reactive nitrogen(nitrogen compounds,etc.)can be con-verted by microbial activities in the natural world,and the transition between simple nitrogen and inorganic compounds is referred to as the nitrogen cycle.This process includes nitrogen fixation,nitrification,denitrification,etc,which mainly occurs through the promotion of en-zymes in nature.With the development of human society,the artificial nitrogen cycle has re-ceived extensive attention and has become an important topic in this present moment.Catalytic reaction plays an important role in the nitrogen cycle process.In this context,we work on the following three aspects for some typical nitrogen cycle processes:1.Design catalysts for electrochemical nitrogen reduction.In this part,the design of the catalyst is carried out from the aspects of single-atom catalysts and alloy catalysts.A series of B-doped graphene edges was firstly designed as electrochemical nitrogen reduction catalysts according to the N₂ active principle of the accept-feedback mechanism,the fermi level of cat-alyst is close to the?*orbital of N₂,which is favorable for the electrons transfer from catalyst to the?*orbital of N₂ to activate N₂.Gra-B₁ has the best catalytic activity for nitrogen reduc-tion with an onset potential of only 0.18 V.The adsorption and activation mechanisms of N₂on the Gra-B_x system were studied by the analysis of DOS and identified the positive correlation between adsorption and activation of nitrogen does not always hold,the Gra-B₁ catalyst works best with activating the nitrogen.The electrons numbers of Gra-B₁ and intermediates during NRR are calculated by population analysis.Through the analysis of charge transfer,it was found that the graphene substrate donated electrons to the adsorbate through B atoms,which is promote the reduction reaction and reduces the reaction energy of the potential-determining step.After that,the reaction mechanism and reaction pathway of the electrochemical nitrogen reduction process on a series of Cu Mn alloys are investigated,in which Cu₁Mn₁ exhibited the best electrochemical nitrogen reduction activity with an overpotential of only 0.42 V.Accord-ing to the results of charge density difference,population analysis,and density of states anal-ysis,it is clear that the electron transfer process from Mn atoms to Cu atoms on alloys en-hanced the catalytic activity of nitrogen reduction at Mn sites.Based on the structure-activity analysis of the above two kinds of catalysts,we believe that the nitrogen reduction activity of the catalyst can be improved by rationally designing in catalyst structure,increasing the num-ber of charges on the active site of the catalyst effectively promoting the electron transfer between N₂ and the active site.2.Design catalyst for electrochemical ammonia oxidation.In this section,Mo doped blue phosphorus supported by Zr C was designed as the perfect electrochemical ammonia oxidation reaction catalyst.Taking the N atom adsorption energy as the descriptor,calculate the reaction energy of AOR elementary reactions on a series of single atom catalysts,and made a graph of volcano curve.Combined with the screening of small molecule adsorption energy,it was found that the Zr C-supported single-atom Mo doped blue phosphorus owned the best ammonia oxi-dation catalytic activity and selectivity,with a calculated overpotential of only 0.46 V.Bene-fiting from the good lattice matching of Zr C and BP,and the strong covalent bond between C and P atoms,this catalyst exhibits good stability.The energy barriers for Mo atoms diffuse to other sites as high as 1.97 e V and 2.13 e V,indicating the good kinetic stability of catalyst at room temperature.The electronic structure analysis shows that the Mo atom exhibits a low density of states below the Fermi level and a high density of states near the Fermi level,and this is the reason that Zr C/Mo-BP exhibits excellent catalytic activity during the AOR process.3.Design catalyst for nitrate reduction to produce ammonia.First,a graphene-supported phthalocyanine doped with transition metals was designed as the catalyst for electrochemical nitrate reduction.Based on the strong?-?stacking interaction between graphene and phthalo-cyanine,the structure has good stability.According to the calculation of Gra/Pc doped with transition metal atom,the adsorption of N*on the catalysts system varies in a large range,indicating that the catalysts have a very considerable application prospect in catalytic reactions.For the screened Gra/M-Pc(M=Cu,Ni,Pt,Zn),investigate the electrochemical catalytic ac-tivity of nitrate was investigated.Among them,the Cu atom-doped catalyst has the lowest reduction potential of-0.07 V vs.RHE.Since the single-atom active site hardly provides the triangular hydrogen atom adsorption sites consisting of three atoms therefore the adsorption of hydrogen and the hydrogen evolution reaction is suppressed,and the catalyst shows satis-factory ammonia selectivity.In addition,in order to further analyze the mechanism of nitrate reduction on Cu,the reaction mechanism of nitrate reduction to produce ammonia on bulk Cu(FCC-Cu)and metastable Cu(including BCC-Cu,HCP-Cu)have been studied in this work.By calculating the nitrate reduction process on the surface of the catalyst,BCC-Cu has the best nitrate reduction activity,with the onset potential of only 0.01 V vs.RHE.Through the analysis of the electronic structure of NOOH*,it is found that BCC-Cu owns the special crys-tal structure,in which the lower coordination number of Cu atoms and the stronger bonding ability of surface Cu.This structural property is in favor of strengthening the Cu-N bond be-tween NOOH*and the catalyst,meanwhile promoting the adsorption of NOOH*,therefore reducing the reaction energy and onset potential of this step.